handbook/zfs: Change zfs send -P into zfs send -p

The handbook refers to the -P flag which according to zfs-send(8) means
(machine) parsable output, but the desired effect is for the properties
to be included in the send.

Change -P to -p, which zfs-send(8) identifies as the flag to include
properties.

Approved by: doc (debdrup), gerald (mentor)
Differential Revision: https://reviews.freebsd.org/D35015